In order to prevent the contents of a bottle from being adulterated or diluted it is necessary to provide a closure which incorporates a one way valve which will allow the contents to be poured fully from the bottle when it is inverted but will prevent liquid from being poured into the bottle when it is upright. It is also necessary for the valve to be protected so that it cannot be held open when the bottle is upright. It is further necessary for the closure to be locked permanently in the neck of the bottle so that it cannot be removed without breaking the bottle. The closure must also be designed so that it is difficult and preferably impossible to cut the valve portion away thereby opening the bottle while leaving portions of the closure locked in the neck.
Many attempts have been made in the past to design a closure which will meet these requirements but so far as is known prior closures have always been susceptible to some form of damage or interference, or they have been expensive to manufacture or they have been difficult to assemble in the bottle neck.